Copy from ZBX-3286 comments:
For instance, how an user can understand that after he changed the Regexp previously discovered and added resources currently are not passing by filter?
So if a gathering the data will be stopped then it could be some "indicator" that the resources are not discovered anymore an they going to be deleted in N days.

How is else the user can understand that some resources are not discovered anymore?

I would suggest to change default value 0 days for "Keep lost resources period (in days)" to for instance 1 day when an user creates new discovery rule. See a reason in the (11) of ZBX-3286